The present invention relates to a wafer polishing apparatus and a wafer polishing quantity detection method or, in particular, to a wafer polishing apparatus and a wafer polishing quantity detection method based on the chemical mechanical polishing (CMP) process used for flattening the surface of a wafer during the process of forming an IC pattern thereon.
In recent years, the IC has been further miniaturized and IC patterns have been formed over a multiplicity of layers. Some unevenness unavoidably occurs in the surface of a layer formed with a pattern. In the prior art, the next pattern is formed directly on such an uneven surface. With the increase in the number of layers, however, the reduced size of line widths makes it difficult to form a superior pattern, often leading to a defect. In view of this, a practice prevails in that the surface formed with a pattern is polished to a flat surface followed by forming the pattern of the next layer. Also, a metal layer for connecting the layers is formed by forming a hole and then plating, and the metal layer on the surface is removed by polishing while leaving intact the portion of the metal layer corresponding to the hole. For polishing the wafer during the process of forming the IC pattern in this way, a wafer polishing apparatus (CMP apparatus) based on the CMP method is used.
FIGS. 1A and 1B are diagrams for explaining the machining process using the CMP method in the fabrication of an IC. FIG. 1A shows the process of flattening by polishing the surface of a layer insulating film, and FIG. 1B shows the process of polishing the surface so that only the metal layer corresponding to the hole portion is left intact. In the case where the layer insulating film 3 is formed after forming a pattern 2 of a metal layer on a substrate 1, the portion of the pattern 2 is higher than the other portions, thereby causing an unevenness in the surface. In view of this, the surface is polished with the CMP apparatus into the state, as shown on the right side, and then the next pattern is formed. Also, when forming a metal layer for connecting the layers, a connecting hole is formed on the pattern 2 of the lower layer as shown in FIG. 1B, after which the metal layer 4 is formed over the entire surface by plating or the like. After that, the surface is polished until the metal layer 4 on the surface is entirely removed by the CMP apparatus.
FIG. 2 is a diagram schematically showing a basic configuration of the CMP apparatus. As shown in FIG. 2, the CMP apparatus includes a polishing stool 11 and a wafer holding head 20. An elastic polishing cloth 14 is attached on the surface of the polishing stool 11. The polishing stool 11 is coupled to a motor 13 through a spindle 12 and adapted to rotate in the direction of arrow. A slurry providing an abrasive is supplied on the polishing cloth 14 of the rotating polishing stool 11 from a nozzle not shown. The polishing cloth 14 may be formed with a groove for facilitating the supply of the slurry to the contact surface with the wafer. The wafer holding head 20 holds the wafer to be polished, and rotates while pressing the wafer against the polishing cloth 14 under a predetermined pressure. As a result, the surface of the wafer held is polished. Although FIG. 2 shows the case in which only one wafer holding head 20 is provided, a plurality of wafer holding heads 20 may be provided on a single polishing stool.
Various types of wafer holding mechanisms are available for the wafer holding head 20. For example, Japanese Unexamined Patent Publications (Kokai) No. 6-79618, No. 8-229808 and No. 10-175161 disclose a wafer polishing apparatus in which a wafer is held by being closely attached to a carrier by adsorption or the like means and the carrier is pushed thereby to press the wafer against the polishing cloth. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 51-90095 discloses a lapping apparatus, not a CMP apparatus, in which a wafer is held by being attached to a carrier with an adhesive or two-side tape or the like and the carrier is pushed to press the wafer against the polishing cloth. Such apparatuses can securely hold the wafer being polished, but in the presence of dust or the like foreign matter between the carrier and the back of the wafer, the pressure of the carrier cannot be uniformly transmitted over the entire wafer surface, thereby making it difficult to polish the whole wafer surface uniformly. In order to solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 1-188265 discloses a lapping apparatus in which a carrier includes an air outlet for applying an air pressure from the back of the wafer and by thus pressing the wafer against the stool, the surface is polished while at the same time keeping the carrier and the wafer out of contact with each other. Also, the present applicant has disclosed in Japanese Patent Publication (Kokai) No. 9-138925 corresponding published application JP-A-11-347918 and U.S. application Ser. No. 09/053,062, a wafer polishing apparatus comprising an air bag for pressing the carrier with an air outlet thereby to facilitate the adjustment of the pressure of the wafer against the polishing cloth in contactless manner.
In the CMP apparatus, the surface of the IC pattern is required to be polished accurately by a predetermined quantity. Various methods have been proposed for controlling the polishing quantity accurately. A method capable of most accurately controlling the polishing quantity is a process control method in which the polishing work is conducted little by little while measuring the polishing quantity. According to this method, in order to secure the required film thickness, the remaining film thickness is measured after every polishing session of several seconds, and if there is any shortage of the polishing quantity, the polishing is repeated. This method, however, is very low in productivity and encounters the problem of difficulty of application to mass production. Another method of controlling the polishing quantity is by controlling the time while stabilizing the polishing process. Due to the variations of the polishing process, however, it is difficult to control the polishing quantity with high accuracy. Also, the use of a dummy wafer for monitoring the relation between the polishing time and the polishing quantity poses the problem of a reduced throughput. Other methods that have thus far been proposed include a method for detecting the capacity with the metal wiring layer under an oxide film and a method in which the torque change is detected taking advantage of the fact that the torque required for polishing varies with the type of the layer. Under the circumstances, however, these methods fail to be satisfactory solutions as the scope of application is limited and the detection accuracy is not sufficiently high.
It is therefore desirable to directly measure the thickness of the wafer being polished and to calculate and control the polishing quantity based on the change in the thickness. Nevertheless, it is difficult to measure the wafer thickness during the polishing operation. Various methods have thus been proposed for measuring the quantity corresponding to the change in the wafer thickness. The patent publication No. 51-90095 described above, for example, discloses a lapping apparatus in which one of the parts constituting a detector such as an electric micrometer is mounted in a sample holder arranged on a lapping stool and the other parts are mounted in a sample holding frame with the work attached thereto thereby to detect the change in the thickness of the work. The sample holder, like the work, is in contact with the lapping stool, and the polishing quantity can be detected by detecting the displacement of the sample holding frame. In this lapping apparatus, however, the sample holding frame is urged by the sample holder in such a manner as to be pressed against the lapping stool, and therefore the force is developed between the sample holder and the sample holding frame with the rotation of the lapping stool or the sample holding frame, thereby posing the problem that the pressure under which the work is pressed against the lapping stool undergoes a fluctuation. Especially, in the case of the CMP apparatus having an elastic polishing cloth on the surface of the stool, the problem is the large vibration of the sample holder caused by the rotation.
In order to obviate the problems mentioned above, Japanese Unexamined Patent Publications (Kokai) No. 8-229808 and No. 10-175161 described above disclose a configuration in which a polishing surface adjusting ring is arranged around the wafer and the fluctuation of the polishing cloth within the ring is reduced to suppress the polishing pressure unevenly distributed to the edge of the wafer. Especially, Japanese Unexamined Patent Publication (Kokai) No. 10-175161 discloses a CMP apparatus in which some parts constituting a detector such as an electric micrometer are arranged on a carrier for holding the wafer and the other parts are arranged on a member adapted for displacement in contact with the polishing cloth between the wafer and the polishing surface adjusting ring thereby to detect the work thickness change. With this apparatus, the force developed between the members displaced by contact between the carrier and the polishing cloth can be substantially ignored. Also, the members are in contact with the polishing cloth only within the polishing surface adjusting ring where the fluctuation is comparatively small. Therefore, an accurate measurement should be possible. In this apparatus, however, the periodic fluctuation developed with the rotation of the wafer holding head and the carrier has a direct effect on the measurement, and therefore the signal is actually difficult to process.
Japanese Unexamined Patent Publication (Kokai) No. 11-198025 corresponding to U.S. application Ser. No. 09/112,287 discloses a wafer polishing apparatus in which an arm passing through the central portion of a carrier is arranged on a polishing surface adjusting ring or on an annular pad inside the polishing surface adjusting ring, and a detector is arranged for detecting the relative vertical displacement between the arm and the central portion of the carrier, thereby reducing the effect of the periodic fluctuation caused by the rotation of the wafer holding head and the carrier.
Even in the case where the change in wafer thickness is detected by the wafer polishing apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-198025, the detection signal undergoes such a sharp fluctuation that it is very difficult to control the apparatus to calculate a correct polishing quantity from this detection signal and to polish only the required polishing quantity.
Also, the actual implementation of this method generates heat by polishing in the portions of the polishing surface adjusting ring and a reference pad in contact with the polishing cloth, thereby changing the temperature distribution in these parts. As a result, the relative positions of the portions supporting the detector or in contact with the detector are changed by thermal expansion, thereby adversely affecting the detection signal. In order to remove this effect, a model indicating the relation between the detection signal and the actual polishing quantity is produced in advance, and in accordance with the model, the detection signal is corrected, so that when the value of the corrected detection signal changes by a predetermined value, it is determined that the polishing is accomplished by a designated quantity.
Another method of controlling the polishing quantity is to control the time by stabilizing the polishing process. In this method, a model indicating the relation between the polishing time and the polishing quantity is produced in advance, and in accordance with this model, the polishing time required for polishing a designated polishing quantity is calculated and the actual polishing work is conducted for the particular polishing time. This method is simple and involves a comparatively accurate polishing quantity as far as the polishing process is stable.
In the above-mentioned method of controlling the time by producing a model indicating the relation between the polishing time and the polishing quantity and the method of correcting by detecting a quantity corresponding to the change in wafer thickness, a model is produced in advance by polishing a reference wafer and measuring the change in wafer thickness before and after the polishing work. In spite of this, the fluctuations of various factors including the temperature and the wear of the polishing cloth makes it impossible to conduct the polishing work as according to the model, thereby leading to the problem of an error in the polishing quantity.
In order to solve these problems, a dummy wafer is polished at the same time or the dummy wafer is periodically polished to correct a model. The use of a dummy wafer, however, poses the problem of a correspondingly lower throughput.
The present invention is intended to solve these problems, and the object thereof is to provide a wafer polishing apparatus in which the polishing quantity can be accurately controlled.
In order to realize the object described above, according to a first aspect of the invention, there is provided a wafer polishing apparatus and a wafer polishing method therefor, in which the detection signal of a detector is sampled with such a sampling period that the number of times samples are taken per rotation of a polishing stool is plural, the sampling data in a number an integer multiple of the number of times sampled per rotation is averaged to calculate the moving average data, whereby the polishing quantity is calculated.
Specifically, a wafer polishing apparatus according to the first aspect of the invention comprises a rotatable polishing stool with a polishing cloth arranged on the surface thereof, a carrier rotated about a rotational axis different from and parallel to the rotational axis of the polishing stool thereby to bring the wafer into contact with the polishing cloth under a predetermined pressure, a pad arranged around the wafer in such a manner as to contact the polishing cloth under a predetermined pressure, a detector for detecting the change in the relative positions of the back surface of the wafer or the carrier and the pad, an operating unit for computing the polishing quantity by processing the detection signal of the detector, and a control unit for controlling the polishing operation in accordance with the polishing quantity computed, characterized in that the operating unit includes a sampling unit for sampling the detection signal of the detector with such a sampling period that the number of times sampled per rotation of the polishing stool is plural, a moving average calculating unit for calculating the moving average data by averaging the sampling data in the number an integer multiple of the number of times sampled per rotation, and a polishing quantity computing unit for computing the polishing quantity from the moving average data.
In the wafer polishing apparatus, the polishing stool and the carrier (wafer holding head) are rotated with predetermined periods, respectively. The polishing stool and the carrier have their respective slight inclination or undulation in their motion. Even in the case where the wafer has a constant thickness, therefore, the detection signal of the detector changes with the rotational period of the polishing stool and the carrier. Specifically, the detection signal changes with a period equal to the least common multiple of the two periods. As far as the two rotational periods is the same, therefore, the same change is repeated with the particular period. In the case where one rotational period is an integer multiple of the other rotational period, on the other hand, the change is repeated with the larger period, while in the case where the two rotational periods are slightly different or an integer multiple of one period is slightly different from the other period, then the change is undulated. Normally, the polishing stool and the carrier are set to the same rotational period, or the rotational period of the polishing stool is longer than that of the carrier and set to an integer multiple of the latter, and therefore the same change is repeated for each rotation of the polishing stool. In the wafer polishing apparatus and the wafer polishing method therefor according to the invention, the detection signal is sampled a plurality of times per rotation of the polishing stool, and moving average data is calculated by averaging the sampling data in a number equal to an integer multiple of the number of times sampled per rotation, and therefore data substantially associated with the actual change of the wafer thickness, with little fluctuation, is obtained.
Even in the case where the moving average data are calculated as described above, however, the slurry is not stably supplied over the entire surface, heat begins to be generated by the polishing and data are not stabilized and fluctuate irregularly for a certain length of time from the polishing start. In view of this, the data are not used before the lapse of a predetermined time from the polishing start, and the polishing quantity is desirably computed from the data obtained after the lapse of the predetermined time. Further, the generation of heat by the polishing continues as long as the polishing operation. Therefore, the temperature distribution of various parts including the carrier, the pad and the arm supporting the detector is changed thereby to change the thermal expansion, so that the relative position of the detector is changed. As a result, the detection signal undergoes a change. Also, the detector itself has a temperature characteristic and the detection signal changes with the change in the temperature of the detector. The change in the detection signal due to these factors occurs continuously from the start of the polishing process. In view of this, the polishing quantity computing unit desirably includes a correction data storage unit for storing the correction data calculated from the polishing quantity computed by polishing a sample wafer and the practical measured values of the thickness of the sample wafer taken by different measuring tools before and after the polishing, and a correcting unit for correcting the polishing quantity computed in the polishing quantity computing unit based on the correction data and outputting the resulting polishing quantity.
The first aspect of the invention, though effective in a polishing apparatus for polishing a wafer fixed on a carrier, is also applicable to a configuration in which a pressure fluid layer forming unit for forming a pressure fluid layer at the back of the wafer is arranged on the carrier and the wafer is pressed against the polishing cloth by the pressure fluid layer.
In the wafer polishing apparatus having a configuration that the wafer is pressed against the polishing cloth by the pressure fluid layer, the following phenomenon has been found to occur during the process of polishing off the metal layer formed on the insulating material layer as shown in FIG. 1B. Specifically, when the metal layer is removed to such an extent that the insulating material layer is exposed in a part of the surface, the detection signal sharply decreases, and the detection signal increases again at the time point when the whole metal layer is removed from the surface. In the case of polishing off the metal layer formed on the insulating material layer, therefore, the time point at which the surface metal layer is removed can be determined by observing the change in the detection signal. Therefore, the metal layer can be accurately removed as shown in FIG. 1B, for example, by polishing a little longer than the time point when the detection signal begins to increase again after the sharp decline.
As described above, a temperature distribution occurs at various parts due to the polishing heat. Desirably, therefore, a temperature sensor for detecting the temperature of the neighborhood of the detector is provided to make a correction according to the temperature characteristic of the detector. Also, another temperature sensor is provided for detecting at at least some of the members between the portion where the relative positions are detected by the detector and the back of the wafer or the portion of the carrier facing the wafer. Still another temperature sensor is provided for detecting the temperature of at least some of the members between the portion where the relative positions are detected by the detector and the portion of the pad facing the polishing cloth. Based on the temperatures detected by these temperature sensors, the difference in thermal expansion between the related portions is calculated and the detection signal is corrected by the difference of the thermal expansion, thereby improving the detection accuracy.
The wafer holding head for holding the carrier rotates. For transmitting the detection signal of the detector and the detection signals of the temperature sensors, therefore, a slip ring is desirably provided for accommodating the transmission path of the electrical signal in the rotational shaft of the wafer holding head. At the same time, an analog processing circuit for the detection signal and an A/D conversion circuit for converting the output of the analog processing circuit into digital data are desirably arranged on the wafer holding head, so that the digital data are transmitted to an external data processing circuit through the slip ring.
According to a second aspect of the invention, there is provided a wafer polishing apparatus in which the actual polishing quantity of a normal wafer to be polished is measured and this practical measured value is compared with a predetermined polishing quantity, so that a model is corrected by the difference whenever required.
Specifically, the wafer polishing apparatus according to the second aspect of the invention comprises a rotatable polishing stool with a polishing cloth arranged on the surface thereof, a wafer holding head for holding a wafer and rotated while pressing the wafer surface against the polishing cloth, and a control unit for controlling the operation in such a manner as to perform the polishing operation by a designated polishing quantity data in accordance with a polishing model, characterized by further comprising a polishing quantity measuring unit for taking the practical measured value of the polishing quantity of the wafer polished, and a polishing model correcting unit for correcting the polishing model in accordance with the difference the practical measured value of the polishing quantity taken by the polishing quantity measuring unit and the polishing quantity data.
According to this invention, a model is corrected as required according to the error of the polishing quantity of not a dummy wafer but a normal wafer, and therefore the polishing quantity can be controlled with an optimum model at a particular time point without reducing the throughput. The polishing quantity can thus be controlled with a high accuracy.
In a polishing quantity measuring unit of one type including a wafer thickness measuring tool for measuring the thickness of a wafer, the practical measured value of the polishing quantity is calculated from the difference in wafer thickness before and after polishing. In a polishing quantity measuring unit of another type including a thickness measuring tool for detecting the thickness of an oxide insulating film formed on the wafer, the practical measured value of the polishing quantity is calculated from the difference in the thickness of the oxide insulating film before and after polishing.
The polishing models include the one indicating the change in the polishing quantity of the wafer with respect to the polishing time as described above, and the one for correcting the output of a displacement measuring tool produced from the practical measured value and the output of the displacement measuring tool used for detecting the change in vertical relative positions of the surface of the polishing cloth and the back of the wafer or the wafer holding head.
In one correction method, a polishing model is corrected in such a manner as to acquire an intermediate value between the practical measured value of the polishing quantity and the polishing quantity data. In another conceivable method, the correction is effected when the difference increases between the practical measured value of the polishing quantity and the polishing quantity data.